Intruder Alarm Circuit

Description

Here is the circuit diagram of an ultra sensitive intruder alarm. The shadow of an intruder passing few meters nearby the circuit is enough to trigger the alarm.

Here IC2 uA 741 is wired as a sensitive comparator ,whose set point is set by R6 &R7. The voltage divide by LDR and R9 is given at non inverting pin of IC2. At standby mode these two voltages are set equal by adjusting R9. Now the output (pin6) of comparator will be high.Transistor Q1 will be off. The voltage at trigger pin of IC1 will be positive and there will be no alarm. When there is an intruder near the LDR the shadow causes its resistance to increase. Now the voltages at the inputs of comparator will be different and the out put of IC2 will be low. This makes Q1 on. This makes a negative going pulse to trigger the IC1 which is wired as a monostable multivibrator.The out put of IC1 will be amplified by Q2 (SL 100) to produce alarm.

Intruder Alarm Circuit Diagram with Parts List

Intruder Alarm Circuit Diagram

Notes

To setup the alarm ,power up the circuit and adjust R9 so that LED D1 goes off.

The LDR can be housed in a dark tube to increase sensitivity.

The sensitivity is very important here. If you cannot adjust the required sensitivity properly, use one LOW resistance (~1K ) POT in series with R9 for fine adjustment.

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Comments

keerthi

September 7, 2015

i have done dis circuit using with IR SENSOR MODULE AND 555 TIMER. what happens if we connect the IR sensor module directly to the buzzer.instead of connecting it to 555 timer of reset pin. plzz help me its urgent..

Excuse me, i connected the circuit and the sensor are working, but the buzzer continue to make sound when shadow are not detected and when shadow are detected the buzzer will not make any sound. Any help are appreciated.

Seetharaman, I have connected the component on vero board and is working perfectly well. Few days later, buzzer continue to sound even when no shadow is on the LDR. I change the LDR but the buzzer kept sounding without sensing shadow. Pls what could be the posible problem?

I have connected the component on bread board correctly and the led will light on,the ldr is working because i have test it on ammeter.but when shadow fall on the ldr the buzzer will not sound and sometimes when already there is shadow on the ldr and i connect it the buzze will sound,suppose to if i remove the darkness also the buzzer to stop the sound what is the problem? Please urgent rescue.

Will any one help me,the circuit was working very good for 2 days after making then after which buzzer is not working what will be the fault i have checked the buzzer ,i have also check the ic it is working because it is triggring .

Hi Rat1 741 can operate from plus minus 2 volts on wards up to plus minus 20 volts. In the above circuit the ref Pin No2 voltage is held at half the supply voltage (9 volt) that is plus minus 4.5 volts.

Seetharaman,
What does k1 written below d buzzer mean?
If i go to the shop to buy the buzzer ,what should i ask for? Also, please suggest a few alternatives for the buzzer K1 incase it’s not available in the market. Please help. Urgent!!

. .i’m an EcE student here I got my best project here in circuits today .. .and my professor are so happy that I have my project . remember the transistor based intercom . ..very simple but it was so great ..thank you

. .hi i’m me . .can you help me find what to replace sL100 and bc157 . . out of the market again …. can you give at least 10 replacement to each transistor …I don’t have much money to spent searching the net . .I’l come back tomorrow . .its about our project please help me. .i’m out ..I will be so thank full if i can find answers tomorrow,,

hi seetharaman! I just wanna ask something. I’ve tried you circuit but i have a big problem, cause when I connect the power supply to the circuit, the alarm was already triggered and the the potentiometer really does not vary the sensitivity of the LED… please help me to trouble shoot my circuit.. THANK YOU!

Hi Batutuy The circuit is correct. When the light falls on the LDR it resistance will be low, hence IC output at Pin no6 will go high. Q1 emitter will go high LED will be in off ststus. When the shadow falls on LDR, its resistance will go high and Q1 will conduct and LED will glow with a negative going pulse to IC1 to produce alarm.